Shot-peening treatments



Jan. 15, 1963 Filed April 3, 1959 U/V Tiff/4 TED J. J. BUSH ETA].3,073,022

SHOT-PEENING TREATMENTS 5 Sheets-Sheet 1 LARGE SHOT) SMALLER SHOT} FAT/6UE L /F E lMFROVEMEN T SUBJEC PROCESS FA776UE L/FE- CYCLES UN TREATED Jan. 15, 1963 J- J. BUSH ET AL 3,

SHOT-PEENING TREATMENTS Filed April 5, 1959 5 Sheets-Sheet 2 PRczwrFAILURE //v POPULA7/0/V 8 & 8 8 8 LIFE CYCLES cfeazye pdlzzir gram/trfield Hills, and

"a high velocity. conventionally,

"a single step process in which the surface of a metal part manner forapredetermined time. peening treatment ordinarily was applied as asingle, conthe fatigue 1 generally to at least twice the a conventionalshot-.peening operation.

United States 3,073,022 SHOT-PEENING TREATMENTS John J. Bush, Royal Oak,Raymond L. Mattson, Bloom- James GeorgeRoberts, Warren, Mich, assignors.to General Motors Corporation, Detroit, Mich, a corporation of DelawareFiled Apr. 3, 1959, Ser. No. 803,961

' 16 Claims. (Cl. 29-553) This invention relates to the treatment ofmetal surfaces and more particularly to a method of increasing thefaitigue life of metal parts.

Hammering or peening is well known to cause small, permanent surfacedeformations which contribute to an increase in strength and hardnessofthe metal of which the part is composed.

In addition to hammering, it has "previously been knownto peen metalsurfaces with small, hardened shot particles which impinge on thesurface at shot-peening has been is subjected to a high velocity streamof shot in a given Heretofore, the shottinuous treatment rather than 'aseries or plurality of shorter treatments of similar intensity. Theplurality of treatments of similar intensity provided no materialbenefit in strength or hardness of the part treated over the benefitsobtained from a single, continuous treatment.

However, we have now found that a plurality of shotpeening treatmentscan be used to materially increase We have unexpectedly found that byusing a plurality of treatments in a specific manner we can increase thefatigue life of a metal part normal life obtained from Inaccordance withour invention a metal surface is first shot-peened in a conventionalmanner and thereafter We have found that when the seca lower intensityand a smaller and features of this invention-will appear more clearlyfrom the following descrip- 'tion'of a preferred embodiment thereof andfrom the .drawings,. Lin"which: I

, {FIGURE 1 shows a diagrammatic view of the essential steps involved inour invention; 1

. FIGURE 2 shows a bar chart which compares the fatigue! life'ofpartstreated by the subject method with the fatigue life of parts which areconventionally peened; Weibull plot of fatigue life of unv eened Yparts,conventionally ,peened parts, and parts FIGURE. 3 is a treated in thesubject'manner;

FIGURE 4 is 'a'Weibull plot comparing single and double peened specimensshowing fatigue life of the effect of lowering theintensity Iota-secondpeening treatment;

plot of fatigue life of single and of reducing and FIGURE 5 is a Weibulldouble peened specimens showing the effect shot size in a second peeningtreatment.

conventionally, shot-peening is practiced by imparting an impetus ,tothe I shot so that the on a given surfacea't a high'velo 'ty,Infonernethod the shot is accelerated to ft he high'velocity by means ofa rapidly rotating wheel orimpeller." The .wheel has a central cavityinto which the shot is introduced. Several life of a metal part to amuch greater degree "than that ever accomplished by means of aconventional, single shot-peening treatment.

shot will impinge atent if desired, by

shot into the wheel passages.

a workpiece is y the methods and apparatus set forth in the SAE Manualwith hardened metal shot,

' coverage desired.

' with SAE 70 chilled iron shot using an intensity 3,073,022 PatentedJan. 15, 1953 iCQ "the. wheel. The shot placed in the central cavity ofthe wheel passes through the passages of the rotating wheel where it isaccelerated to a suitable velocity whereupon it isdischarged from theopenings on the'circumferential perimeter ofthe wheel. The shot can beemitted from the wheelin the form of a single, unidirectional'stream,employing a control device in the central cavity of the wh'eeltoregulatethe point of entry of the The particular size of shot used,intensity of the shot blast, duration of peening, etc. are variable, andthe preferred treatment to be used will depend upon the composition,hardness, configuration, etc. of the specific item being treated.However, typically conventionally treated in accordance with onShot-Peening, SP-84.

Conventional shot-peening is usually accomplished such as caststeelshot, cut steel wire shot or cast'iron shot having a mean diameterof from about 0.005 inch to about 0.13 inch, andin some instances a.shotsize as low as about 0.002 inch can be used. The duration of theshot-peening treatment is variable, depending upon the intensityemployed and the For example, a heretofore satisfactory shot-peeningtreatment for hot-rolled SAE 5160. steel springs which are hardened. andtempered to Rockwell C 48 hardness involves shot-peening to full visualcoverage of about a-0.00"20C SAE intensity.

, creases in fatigue life when ver a cess with strength are obtained'size or the intensity of the secondary treatment, we have Thisinvention comprehends subjecting previously shotwhich In order to obtainthe substantial the second shot-peening treatment should be of an SAEintensity which is lower than that initially used or a smaller shot sizeshouldbe marked improvements in fatigue by singly varying either theshot employed. Although experienced the most significant success inobtaining inboth the shot size and intensity are lowered in thesecondary treatment.

m The optimum intensity and shot size for each treatment, of course,isdependent upon thenature of the part being have experiencedconsiderable succonventional However, we

our invention using an optimum,

j shot-peening treatment for the first step and an SAE intensity andshot size which is approximately to that whichis used in the initialtreatment. The wordfintensity is used herein to describe the nature ofa'shotblast in accordance with the-normal andaccepted practice asdescribed in the Society of Automotive Engineers 1956 Handbook. 'Theterm intensity thereforerefers to the .v the properties of the blast,e.g., velocity,,size,

usity, kind, of material andzhardness .of the shot. tionally,

posurelto the blast, e.,g., length of time, and shot flow rate.

fused to accomplish the designated as having similar intensities.

eflect produced on a standard-test specimen by a shot peening treatment.The basis of measurement of SAE intensity is the measurement of thedegree of curvature of an'initially fiat steel test strip after the teststrip is subjected to as'hot-peening treatment. The extent of thiscurvature on the standard test sampleafter full visual coverage servesas a measurement of. the intensity of the blast, U The degreeofcurvature or intensity depends upon shape, den- Add the curvaturedepends upon the properties ofexangle of impact Thus, varying shotblasts can: ,be samefeffect and are, thefefore,

In practicing 'our invention we; have found that, in general, v,anyreductionin the -intensitygerishot;size of the second treatment from,the intensity of the initial,

aotaoa'a conventional peening treatment contributes to an increase infatigue life of the metal part. The optimum intensity and shot size ofthe second treatment is primarily dependent upon the intensity of thefirst peening treatment and the nature of the material. However, it hasbeen established that when the spring steel surface is initiallyshot-peened in a treatment of approximately 0.016A to approximately0034A SAE intensity, highly satisfactory increases in fatigue life canbe obtained if the secondary treatment is of an SAE intensity of about0.003A to about 0.011A (0.001C intensity=0.0035A intensity).

The nature of shot used to practice our invention in an economical,commercial manner is as pertinent as in conventional peening treatments.For example, this invention can be practiced using cut steel wire shot,condi: tioned cut steel wire shot, cast iron shot and cast steel shot.

Due to the difference in physical properties in various types ofmaterials which may be treated in accordance with our invention, theindividual responses of a number of different materials to ashot-peening treatment is quite variable. Moreover, similar materialsmay even exhibit varied responses to a shot-peening treatment due todifferences in hardness and the like. Accordingly, it is difficult toestablish an optimum treatment which will provide the most beneficialincreases in fatigue life for all materials using our invention. This isparticularly true with respect to the size of shot which is preferablyemployed in the first and second peening treatments. However,satisfactory results are obtainable when a metal part is initiallyshot-peened in the known and accepted manner for such treatments toprovide optimum fatigue resistance and then shot-peened in asecond'treatment using a smaller shot size. We have found that the shotsize for the second treatment generally is preferably approximately /3to /s the size of the shot used for the initial peening treatment. Ingeneral, satisfactory results are obtainable when spring steel, forexample, is initially shot-peened using a shot having a mean diameter ofabout 0.023 inch to 0.066 inch and thereafter subjected to a secondpeening treatment in which the mean diameter of the shot is about 0.007inch to 0.011 inch in size.

Serving as a specific example of the practice of our invention,leaf-spring specimens were formed of hot-rolled SAE 5160 spring steel.These specimens were finished to a rectangular configuration of 0.192inch in thickness, 1.5 inch in width and 12 inches in length. Thefinished specimens were then hardened and tempered to a Rockwell C 48hardness and then shot-peened in accordance with the invention. Afterthe shot-peening treatment, each of the specimens was fatigue tested bysubjecting the shot-peened side to a uniform bending tensile stress overthe central six inches of length with a range of zero to 200,000 poundsper square inch at the surface in each cycle.

In the table immediately specimens were subjected following, a number ofsuch to each of the treatments described. FATIGUE TEST RESULTS PrimaryTreatment Secondary Treatment Mean Group Life 3 SAE SAE SAE SAE ShotSize Intensity 2 Shot Size Intensity 1 Chilled iron shot used. 1Commonly designated by deflection in thousandths of an inch of a in ftbtnan 1 b r 1 um er 0 g eye as e ore com ets ru ture. No peening. p p

The above table further shows the relative diflferences in mean lifeproduced by varying treatments which include a single peening and adouble peening treatment in which larger shot and a larger intensitytreatment are used in the second step.

7 We have also found that an increase in fatigue life is also obtainedby grit blasting the surface of a previously shot-peened metal part. Wehave now also found that grit blasting can be used as a secondarytreatment for conventionally shot peened parts. Major increases infatigue life of metal parts can be produced by grit blasting apreviously conventionally peened part. Although improvements in fatiguelife are obtained in this manner, the overall results obtainabletherewith are not as satisfactory as those obtained from the previouslydescribed method.

In general, we have found that an air blast carrying metallic particleshaving an average mean diameter of from about 0.003 inch to 0.017 inchor an SAE grit number of from about G-200 to about 6-40 can be used.These particles can be directed onto the previously shotpeened surfacewith a conventional grit blasting apparatus, such as that commonly usedin the art, employing an air pressure of approximately 70 pounds persquare inch to approximately pounds per square inch.

More specifically, a metal leaf-spring specimen generally similar tothat previously described, was subjected to shot-peening treatments ofan SAE intensity of approximately 0.009C to 0011C using SAE 660 chillediron shot. Following this shot-peening treatment the specimen was gritblasted with a conventional grit blasting apparatus with an SAE G-80grit and an air pressure of approximately 70 pounds per square inchunder an exposure of about 15 seconds. This treatment of the metalleaf-spring provided more than a 100% increase in the fatigue life ofthe leaf-spring specimen.

The beneficial results obtainable with our invention are moreparticularly indicated by the bar chart in FIGURE 2 and the graph shownin FIGURE 3. FIGURE 2, for example, compares fatigue life cycles ofuntreated metal parts, conventionally peened metal parts, and metalparts treated by the method of our invention. While untreated parts havea median fatigue life of less than 100,000 cycles, parts treated inaccordance with our invention display a median fatigue life of over350,000 cycles.

Although the chart of FIGURE 2 clearly indicates the large improvementin fatigue life resulting from the use of our invention, it is notentirely satisfactory. Of great interest are the probability of failureafter testing for x stress cycles and the accuracy with which we canestimate probability.

Information concerning these points is obtained using satisticaltechniques previously described by Johnson (The Median Ranks of SampleValues in Their Population with an Application to Certain FatigueStudies, Leonard G. Johnson, Industrial Mathematics, vol. 2, 1951;Fatigue Tests Proved by Three Statistical Checks, Leonard G. Johnson,SAE Journal, March 1958, pp. 72-73; Statistical Estimation of theMinimum Life in Fatigue, Leonard G. Johnson, G.M. Research LaboratoriesTechnical Memorandum 34-948, March 10, 1958; Statistics of Extremes, E.J. Gumbel, Columbia University Press, 1958, p. 302). Briefly informationwas found on the above points from Weibull plots and confidence bands,respectively. A Weibull distribution function is of the form populationoccurring at some The minimum fatigue life (a) and the median populationcurves used in the Weibull plots of FIGURES 3 through 5 are those thatbest fit the observed fatigue lives. G oodness-of-fit numbers given inFIGURES 3 through 5 are an index to the fit, with a goodness-of-fitnumber=l describing a condition wherein all (x-a) quantities fall on astraight line. The slope (b) of the best fit straight line is known, asis the characteristic life 0, and the Weibull median population line iscompletely determined.

FIGURE 3 is a Weibull plot comparing the fatigue lives of the testgroups noted in Table I. With percent failure in-population,

1F(:c) as ordinate and life cycles. (x) as abscissa, the Weibull plotsinFIGURES 3, 4 and 5 clearly illustrate the increase in life at differentsurvival levels'by our invention.

Table I I Good- (a) (b) Median Test Group Ilei S-lf- Cycles Slope Lite lNon-Peened 0. 90500 7.76 22,720 Single Peened: SAE 660 at 0.00000 0.98191 40,000 2.27 80,730 Double Peened: SAE 660 at i 0.00900; SAE

A Weib-ull plotofthe test groups listed in Table II is shown in FIGURE4. This graph indicates that merely lowering the intensity of thesecondary treatment serves to increase median. 1ife..

Table II Good- (a) (b) Median Test Group ness-of- Cycles Slope Ife FitSingle Peened: SAE 660 at 0.0090 0.98191 40, 000 2.27 80,730 DoublePeeued: SAE 660 at 0.00000; SAE 660 at-0.00l6C. 0.97183 20,000 5.42121,000

The Weibull plot in FIGURE is a graph comparing fatigue lives of thetest groups listed in Table III. This Weibull plot shows that by solelyreducing shot size in the secondary peening treatment, material benefitscan be obtained.

Table 111 Good- (41) (0) Median Test Group negs-gf- Cycles Slope LifeSingle Peened: SAE 660 at 0.0073 0.82292 0 3.32 135,400 Double Peened:SAE 660 at 0.0072A; SAE 70 at 0.0070A- 0. 90017 100, 000 1. 06 149, 300

The 90% confidence bands, also shown on the Weibull plots of FIGURES 3through 5, estimate the boundaries which enclose the middle 90% of allfatigue lives. Confidence interpolation indicates that the median lifeat 1% or 50% failure in population level is significantly improved overa single (conventional) shot-peening treatment by:

(1) Secondary peening by using smaller shot and lower intensity thanused for the primary shot-peening treatment.

(2) Secondary peening by secondary peening alone.

(3) Secondary peening by secondary peening alone.

using reduced intensity for using smaller shot size for Specification ofshot size and intensity for the secondary shot-peening treatment toproduce greatest fatigue life is difficult because of the paucity ofsystematicfatigue data. Nonetheless, some boundaries appear ratherclearly. For optimum results on spring steels, for ex; ample, theprimary shot size should be between 0.023 inch to 0.066 inch in diameterandthe intensity should be between 0.0l6A to 0.034A, the secondary shotsize should be between 0.007 inch to 0.011 inch in diameter, and theintensitybetween 0.003A to 0011A.

It is to be understood that although this invention has been describedin connection with certain specific examples thereof, no limitation isintended thereby except as defined in the appended-claims.

We claim:

1. A method of increasing the fatiguelife of a metal part whichcomprises applying an initial particle blast treatment to a surface of ametal part and thereafter further subjecting said surface to anotherparticle blast of a lesser intensity and smaller particle size than wasemployed in said initial'particle blast treatment.

2. A method f increasing the fatigue life of a metal part whichcomprises applying an initial particle blast treatment to a surface of ametal part and thereafter further subjecting said surface to anotherparticle blast of an intensity and particle size which is approximately/3- to /5 that employed in the initial particle blast treatment.

3. A method of increasing the fatigue life of a metal treatment to asurface of a metal part and thereafter further shot-peening said surfacewith a treatment involving a lower intensity and smaller shot size thanwas employed in said initial shot-peening treatment.

4. A method of increasing the fatigue life of a metal part whichcomprises applying a particle blast treatment to a surface of a metalpart and thereafter further subjeoting said' surface to a particle blasthaving, a lesser intensity than the intensity of said former particleblast treatment.

5. A method of increasing the fatigue life of a metal part whichcomprises applying a particle blast treatment to a surface of a metalpart and thereafter further subjecting said surface to a particle blasttreatment having an intensity which is approximately /3 to /5 theintensity of said former particle blast treatment.

6. A method of increasing the fatigue life of a metal part whichcomprises applying a particle blast treatment to a surface of a metalpart and thereafter subjecting said surface to another particle blasttreatment having an intensity of approximately /3 to /5 that of saidformer particle blast treatment and in which particles are used whichare approximately /3 the size of those used in said former particleblast treatment.

7. The method of increasing the fatigue life of a metal part whichcomprises conditioning the surface of a metal part by inducing acompressive stress thereon with an initial shot-peening treatment andthereafter further conditioning said surface with a shot-peeningtreatment having an intensity which is lesser than that of said initialshot-peening treatment 8. The'method of increasing the fatigue life of ametal part which comprises conditioning the surface of a metal part byinducing a compressive stress thereon by an initial shot-peeningtreatment and thereafter further conditioning said surface with ashot-peening treatment using a shot size which is'smaller than thatemployed in said initial shot-peening treatment.

9. The method of increasing the fatigue life of a metal part whichcomprises conditioning a surface of a metal part by inducing acompressive stress thereonwith a shot blast treatment and thereafterfurther conditioning said surface with a shot blast treatment having anintensity of approximately /3 to /5 that of said former shot blasttreatment.

10. The method of increasing the fatigue life of a metal part whichcomprises conditioning a surface of a metal part by inducing acompressive stress thereon with a shot blast treatment and thereafterfurther conditioning said surface with a shot blast treatment having anintensity 'of'approximately /3 that of said former shot blast treatmentand in which the shot used is approximately /3 to /s the size of thatused in said former shot blast treatment.

11. The method of increasing the fatigue life of a metal part whichcomprises shot peening a metal surface with a blastv of shot from theclass consisting of cut wire shot, conditioned cut wire shot, cast ironshot and cast steel shot and thereafter further shot peening said surisof an intensity of approximately /3 to /5 that of said former shot blasttreatment. and the shot used therein is approximately 6 to /s the sizeofthat used in said former shot blast treatment.

12. The method of increasing" the fatigue life of a metal partwhichfcomprises shot-peening a surface .of a

' metal part with a blast of shot from the class consisting meandiameter of shot used is about 0.007 inch to 0.011

inch.

13. The method of increasing the fatigue life of a metal part whichcomprises shot-peening the surface of a spring steel part with shothaving a mean diameter of about 0.023

inch to about 0.066 inch, wherein said shot-peening treatment is of anintensity of about 0016A to 0.034A, and thereafter further shot peeningsaid surface with shot having a mean diameter of about 0.007 inch to0.011 inch, wherein said shot-peening treatment is of an intensity ofabout 0.003A to 0.011A.

14. The method of increasing the fatigue life of metal parts whichcomprises shot-peening the surface of a metal part and thereafter gritblasting said surface of said metal part.

15. The method of increasing the fatigue life of a metal part whichcomprises shot-peening a surface of a metal part to approximatelycoverage and subse quently grit blasting said surface with grit having amean diameter of approximately 0.003 inch to 0.017 inch.

16. The method of increasing the fatigue life of metal parts whichcomprises shot-peening a surface of a spring steel part with shot fromthe class consisting of cut wire shot, conditioned cut wire shot, castiron shot and cast steel shot having a mean diameter of about 0.023 inchto 0.066 inch,wherein said shot-peening treatment is of an intensity ofabout 0.016A to 0.034A, and thereafter grit blasting said surface of ametal part with grit having a mean diameter of approximately 0.003 inchto 0.017

inch.

References Cited in the file of this patent UNITED STATES PATENTS PabensMar. 29, 1932 Palm Jan. 23, 1934 Vorwerk Feb. 6, 1934 Vorwerk Feb. 20,1934 Minich Apr. 20, 1937 Wallace Feb. 14, 1944 Connor Apr. 17, 1945

1. A METHOD OF INCREASING THE FATIGUE LIFE OF A METAL PART WHICHCOMPRISES APPLYING AN INITIAL PARTICLE BLAST TREATMENT TO A SURFACE OF AMETAL PART AND THEREAFTER FURTHER SUBJECTING SAID SURFACE TO ANOTHERPARTICLE BLAST OF AT LESSER INTENSITY AND SMALLER PARTICLE SIZE THEN WASEMPLOYED IN SAID INITIAL PARTICLE BLAST TREATMENT.